Identifying Mail Problems
Mail problems tend to result from one of several common problems:
DNS errors (e.g. a name server claims that a host is the MX for a domain, while it is not).
Relay errors (e.g. a host that should allow accepting email for a certain domain, reject mail sent to that domain).
Invalid address (i.e. we try to send email to an address that really does not exist, or to a user that is not defined on some host).